1. Field of the Invention
The invention relates to optical amplification devices, and more particularly to optical amplification devices with uniform gain intended to be used in wavelength division multiplex optical fiber transmission systems. The amplifier devices are used at regular intervals to compensate the loss on the line. The amplifier devices are preferably identical over the whole of the connection and, for a given input power, preferably have a gain that is as flat as possible over the whole of the wavelength range used in the transmission system.
2. Description of the Prior Art
The gain of an amplifier device differs as a function of the wavelength of the signal to be amplified. In a network including multiple amplifiers, the gain differences between the various frequencies accumulate. Some frequency channels are therefore penalized. To provide a good quality of service for the penalized channels, a first solution is to move the successive amplifiers closer together. A greater number of amplifiers is then used and the cost of the network is consequently high.
Techniques for obtaining a uniform gain over the frequency spectrum from 1530 to 1630 nanometres are known in the art. Using gain equalization filters, modifying the doping profile of the fiber, or using fiber claddings of specific materials make the gain uniform to within 1 dB over the 1530–1560 nm spectrum. However, these techniques can achieve this kind of gain value only for a nominal input power. If the input power deviates from the nominal value, the gain uniformity falls off rapidly. Factors that vary the input power of an amplification device include aging or repair of the fibers, which generates interamplifier attenuation, a change to the network topology upstream of the amplification device, a change to the number of input channels of the amplification device, or a break in the fiber on the upstream side of an optical add and drop multiplexer.
An optical amplification device described in the document U.S. Pat. No. 6,257,329 comprises an optical amplifier, a variable optical attenuator (VOA) at the input of the optical amplifier, and a controller (CONT) controlling the variable optical attenuator and the optical amplifier. The controller is connected to a high-speed information network (IN) so that each node of the network can store the topology of the network. The high-speed information network sends the controller data concerning the operating parameters of the network, such as the interamplifier attenuation, also known as the span attenuation, and simply referred to as span hereinafter. The controller monitors in particular the input power (PIN) and the output power (POUT) of the amplifier.
A variation of the interamplifier attenuation or a modification of the network topology upstream of the optical amplifier (with no change in the number of channels) is declared to the controller by the information network (IN). The controller then knows the magnitude of the input power variation. The controller imposes variable optical attenuator corrections aimed at maintaining the power at the input of the optical amplifier at its nominal value. The optical amplifier therefore provides an output power, a noise factor, and a uniform gain that are approximately constant. The high-speed information network reports to the controller any modification to the assignment or the number of channels at the input of the optical amplifier. The controller then acts on the optical amplifier to maintain its gain constant.
The above optical amplification device and its method of operation have drawbacks. The control method is slow. It is dependent on information transmitted to the controller by the high-speed information network. In the event of a sudden change to the number of channels, resulting for example from accidental cutting of the optical fiber or failure of the optical add and drop multiplexer (and thus inherently unpredictable), correction of the optical amplifier by the controller is possible only after receiving information sent by the high-speed information network. The optical amplifier therefore functions with an input power differing greatly from its nominal power for a time period exceeding several hundred milliseconds. All of the optical amplifiers for which the number of channels has been modified accumulate a gain error during this time period. This causes transitory degradation of service. The controller waits for confirmation from the high-speed information network that the variation is caused by a change in the number of channels before applying a correction to the variable optical attenuator.
Moreover, the reduction in the number of channels at the input of the optical attenuator implies a reduction in the power at the input of the amplification device. For example, there is an input power drop of 15 dB on changing from 32 channels to one channel at the input of the above kind of optical amplification device. Maintaining the gain of the optical amplifier uniform in the event of this kind of power reduction degrades the noise factor (NF), the optical signal to noise ratio (OSNR) of the optical amplifier, and the uniformity of the gain.
Moreover, once the variable optical attenuator has applied the correction, the variable optical attenuator maintains a constant power at the input of the optical amplifier. This degrades the signal/noise ratio at the output of the amplifier and occurs in particular if the variable optical attenuator must reduce the power ahead of the input of the optical amplifier because the signal/noise ratio S/B (expressed in dB) obeys a law of the type S/B=PIN−Nf+C (where PIN is the power in dBm at the input of the amplifier, Nf is the external noise factor, and C is a constant). This implies moving the successive amplifier devices closer together, with an associated cost increase.
There is therefore a need for a method of operating an amplifier device removing one or more of the above drawbacks.